The present invention relates to a process for the production of thin-film silicon transistors on an insulating substrate. It is more particularly used in the production of electronic components with a large surface area. Such components are used in the construction of flat display screens, electronic retinas, active sensors, electronic memories, etc.
A thin-film transistor (TFT) is an insulated grid field effect transistor. It is similar to a MOS transistor (metal-oxide semiconductor) with the difference that it is produced on an amorphous substrate and not on a monocrystalline silicon wafer. As they are not limited by the size of the crystalline substrate, TFT circuits can have very large dimensions.
The TFT on an insulating substrate has been investigated in three different ways:
Way 1: Prior to the advent of MOS technology on silicon, a large amount of research dealt with the production of TFT's on the basis of binary compounds (II-VI), such as CdS and CdSe. They essentially consisted of thin film polycrystalline semiconductors, the TFT being obtained by vacuum masking. Recent variants have been proposed, including photoengraving stages. One of these variants consists of uniformly depositing four layers, respectively of insulating, semiconductor, insulating and metallic materials, followed by the photoengraving of the three latter layers.
Way 2: A second way, which is less than 5 years old, uses silicon monocrystals on an amorphous substrate, with the aim of increasing the compactness of the integrated circuits and if possible to obtain a three-dimensional form. Silicon recrystallization is brought about by laser action and various means are used for obtaining a monocrystal or polycrystal with large grains. Once the silicon has been recrystallized, the process used for producing the transistor is identical to MOS technology, namely implantation, diffusion, oxidation, etc. In all cases it is a high temperature process, which is incompatible with the use of an inexpensive large-size substrate, such as glass. Thus, the field of application is limited to microelectronics.
Way 3: Recently, research has been directed at producing TFT's made from hydrogenated amorphous silicon for producing flat liquid crystal screens. These TFT's are produced by stages involving photoengraving processes from a low temperature hydrogenated amorphous silicon deposit (250.degree. to 300.degree. C.). However, at this temperature, the silicon has a very high hydrogen level (10 to 20%), which is highly prejudicial to the possible recrystallization of the silicon. This recrystallization is necessary if it is wished to produce all the control circuits of the screen and in particular peripheral shift registers (video signal addressing circuit).
In addition, these three ways have the following disadvantages:
Way 1: The composite semiconductors (II-VI) are not very reliable, vacuum masking being limited in resolution and flexibility, whilst photoengraving processes are complex. Reference can be made in this connection to the article by LUO, published in "IEEE Trans. on Electr. Devices", vol. ED 27, no. 1, January 1980.
Way 2: High temperature processes are limited to noble and expensive substrates, such as silica substrates (cf. the article by D. K. Biegelsen et al., published in "Appl. Phys. Lett", 38, no. 3, Feb. 1st 1981).
Way 3: The structure proposed is relatively complex and there is a lack of compatibility with the recrystallization of the silicon. Reference can be made in this connection to the article by Matsamura, published in the "Proceedings of the 12th Conference on Solid State Devices", Tokyo 1980, published in "Japanese Journal of Appl. Phys.", vol. 20, 1981, supp. 20E, pp. 311-314.